In telecommunication, high speed serial links (SerDes) transmit data over various physical media such as copper cables, backplanes, optical fibers, etc. High rate communication channels suffer many problems negatively affecting the integrity of the signals. A primary problem is intersymbol interferences (ISI), defined as a form of distortion of a signal in which one symbol interferes with other symbols in a similar effect as noise, thus making the communication less reliable. ISIs are usually caused by multipath propagation or the inherent non-linear frequency response of a channel whereby causing successive symbols to “blur” together.
The presence of ISIs in the system introduces errors in the decision device at the receiver output. Therefore, in the design of the transmitting and receiving filters, the objective is to minimize the effects of ISIs, and thereby deliver the digital data to its destination with the smallest error rate possible. Some receivers mitigate the effects of ISIs using one or more equalizers, typically feed-forward equalizers (FFEs) and decision-feedback equalizers (DFEs). FFEs can mitigate precursor ISIs and postcursor ISIs, while DFEs can only mitigate postcursor ISIs. Some of the precursor ISIs can be mitigated by a fixed continuous time linear equalizer (CTLE) which operates to compensate for the channel distortion such that the eye in the eye diagram is open enough for the clock and data recovery (CDR) logic to recover the clock and data.
Sometimes, a significant precursor ISI at h(−1) still cannot be compensated which limits the resultant signal-noise ratio (SNR). A conventional approach to reduce the effect of the precursor ISI is to advance the phase of the clock until the precursor level of h(−1) is negligible. However, advancing the clock phase is only performed once and so it is not adapted to any variation of the precursor channel response over time. Also, although the overall performance is improved, advancing the clock phase once causes some degradation of the received signal power of the main cursor and raises the level of the first tap weight of the decision in the DFE.
Generally speaking, conventional FFEs utilize multipliers for analog signals which make them difficult to implement. In contrast, DFEs are relatively easy to implement because they use multipliers for digital input.